This invention relates to an impact drilling tool and, more particularly, to a pneumatically actuated impact drilling tool for rotary drilling utilizing a hammer which strikes an anvil to create an impact force on the drill bit.
Prior to the present invention there have been many types of pneumatically operated impact drilling tools. U.S. Pat. No. 3,616,868 represents a prior version of a fluid actuated impact tool by the present inventor. Another example of a pneumatically operated impact tool can be found in U.S. Pat. No. 3,826,316 issued to Ross Bassinger. Within the prior art there are many types of pneumatically operated devices to create an impact on the drill bit. The drill bit is normally attached to an anvil that is hit by a reciprocating hammer.
To provide a reciprocating hammer that is pneumatically operated within a rotary type impact drilling tool, several valving functions must be performed. For situations involving loss of pressure, a check valve must be incorporated within the impact drilling tool to prevent a reverse flow of fluid (air) within the impact drilling tool thereby drawing cuttings within the moving parts of the tool. A valve mechanism must also provide a means for pressurized air to exert force on the bottom of the hammer thereby raising the hammer above the anvil. Other valving functions must provide a means for exhausting the pressurized air which raises the hammer and to provide pressurized air for driving the hammer against the anvil. The pressurized air above the hammer must be exhausted before the repeating of the next cycle.
In pneumatically operated percussion type rotary drilling tools, the drill rate of a standard drill bit using standard pressure becomes the key to the success of the percussion tool. However, increased drilling rate cannot be accomplished at the expense of destroying the drilling equipment, namely the drill bit. It has been found in the past that a standard holddown force can be applied to rotary drill bits with an impact force being superimposed thereon to greatly increase the rate of drilling. It has also been found in the past that if the impact force can be increased and the holddown force decreased the drill rate can be increased without damage to the drilling equipment. Since the pressure of the pneumatic fluid is normally fixed at a given level, the downward impact of the hammer is dependent upon the upper surface area the fluid is acting against, the stroke length of the hammer and the time required for pressurization and exhaust.
A typical example of a pneumatically operated impact drilling tool that is being commonly sold today is U.S. Pat. No. 3,503,459. This particular patent has numerous limitations including weak structural walls of the casing, very expensive to manufacture, much smaller downward force due to a small surface area which the downward pressure acts against and slow pressurization and exhaust. Any undercut or passage through the casing of an impact drilling tool seriously weakens the lateral strength of the tool, especially for small diameter tools. The patent mentioned in this paragraph is especially weak in the outer casing which may result in bending and subsequent failure.
Various types of percussion drilling devices have been designed and patented where the entire upper diameter of the hammer element is acted upon by the pressurized fluid to drive the hammer downward against the anvil. However, to perform the necessary valving functions each of these devices requires undercuts in the casing with cross bores, slots, undercuts and/or vertical feeds being necessary within the hammer element itself. To insure against structural damage of the hammer element each of these bores, cross slots, undercuts, etc., must end in a rounded surface to prevent structural fatigue and subsequent breaking of the hammer. All of these problems result in decreased strength of the hammer, increased expense of manufacturing and decreased lateral strength of the impact drilling tool.